Biotic and Habitat Index Development

River Studies has worked for the past two decades to develop biological and habitat assessment tools that will aid in conserving the state’s streams and rivers. A primary tool has been the Index of Biotic Integrity (IBI), which is used to measure the health of fish assemblages in streams and rivers. Various aspects of a fish assemblage are ranked relative to benchmarks for similar sized streams and a total score calculated. Those scores can then be compared to those for other streams.

IBI was originally developed in the upper Midwest, but has since been tailored to other geographic areas in the United States and elsewhere. The vast geography of Texas and its diverse climate, topography, and geology have resulted in regionally distinct fish assemblages and any biotic index needs to recognize that variation. As a result, River Studies developed regionally based IBI metrics and criteria. Those “regional” IBIs are used to evaluate aquatic life in wadeable streams and rivers and the Texas Commission on Environmental Quality (TCEQ), the State’s water regulatory agency, has applied them as tools used for surface water quality standards implementation. TPWD has continued its research by evaluating the variability of those metrics and criteria. To be useful, any index should be consistent over time if no changes in habitat and water quality occur. However, since fish communities are dynamic some variation is inevitable and quantifying that variation allows thresholds to be established for determining biological impairment. Such an approach provides some confidence to regulatory agencies, since fish assemblages can be assessed with a known level of precision, making it less likely that a stream will be ranked incorrectly.

Work is underway to refine a habitat quality index for use in relating biological data with physical habitat. As noted, Texas has wide variations in topography and climate that manifest themselves in its diversity of rivers and streams. However, the primary habitat quality index currently used for water quality assessments doesn’t relate well to that diversity and uses measures that favor larger substrates and the typical riffle-run structures that are not common in East Texas streams. To address that issue, we are working with a dataset that includes 91 streams in the eastern half of the state and was collected using detailed methods that include variables relating to geomorphology, in-stream cover, substrate, riparian canopy, and anthropogenic influences. The initial dataset included more than forty habitat variables and we have screened that down to approximately 12 that relate to biological assemblages. We are now developing scoring criteria for those variables. The ultimate goal is to establish scoring ranges that correspond with aquatic life use designations within the Texas Surface Water Quality Standards.